The present invention is, in general, directed to a method of an apparatus for producing magnetic recording media.
The use of magnetic layers or magnetic strips for storing and recording information and other data has become widespread. Several techniques are known for preparing magnetic recording media. Typically, they include applying a magnetic coating containing magnetic or magnetically permeable particles dispersed in a curable binder on a tape or disk surface. For giving the particles a preferred directional orientation they are passed through a magnetic orienting field.
In the production of magnetic recording media it is, however, advantageous to have the magnetic recording layer as smooth as possible. In other words, the magnetic recording layer should be free of indentations, irregularities or undulations. These irregularities would in effect create spacing variations between the magnetic head and the recording medium. Such variations are significant insofar as the signal output will correspondingly vary. These surface variations or fluctuations, as noted, manifest themselves as amplitude variations or fluctuations in the recorded or reproduced signals. Such variations are particularly objectionable insofar as they diminish the sensitivity of the recording media.
Moreover, since a smoother surface on the magnetic recording layer reduces the spacing between the head gap and the layer, the recording head can be spaced closer to the recording surface. This means that the head can be used more successfully in conjunction with higher density recording media. Furthermore, a smoother surface tends to reduce rather significantly the mechanical wear on the head.
There can be several causes for the presence of such irregularities in the recording media. For example, the material used therefor may have fillers which create protrusions. Another cause for surface irregularities such as streaks, undulations or gouges, arises from the coating operation itself. Aside from the foregoing there is, of course, the problem of dust and other particles becoming imbedded in the coating while the media is being formed.
To remedy the foregoing disadvantages a number of approaches have been set forth. One rather common technique for minimizing the surface irregularities is to calendar the coated web. While calendaring, in general, is a satisfactory operation it nonetheless suffers from certain drawbacks. The most significant being the fact that the calendaring rolls themselves are not as entirely smooth as could otherwise be desired. This is because the calendaring rolls themselves have protrusions or gouges which create complementary shaped surface irregularities. In fact even with calendering only a certain proportion of the protrusions in the layer are smoothed or leveled out. In this regard larger and harder protrusions in the magnetic layer only partially undergo elastic deformation during calendaring and are not smoothed after the coated web has passed the calendar nip.
Another way of improving the surface properties of recording media is to pass the coated flexible substrate over a resilient supporting cylinder and burnish the surface of the magnetic coating with an abrasive cylinder. This approach is, however, less than ideal inasmuch as the surface frequently exhibits scratches because of the abrasive cylinder used. An example of such a burnishing process is described generally in U.S. Pat. No. 3,943,666. Besides the foregoing drawbacks burnishing tends to increase the roughness of the surface and create abrasive dust which must be cleaned periodically. An example of an improved burnishing process is set forth in U.S. Pat. No. 4,254,585.
Still another approach for attempting to provide a smooth finish to magnetic coating is essentially a coating transfer method. In such a method, a magnetic coating is applied to a temporary support or release sheet. Then the applied coating is, through a process of lamination, applied to a permanent substrate. Following this the release sheet with the substrate and the magnetic coating therebetween have pressure applied thereto. Subsequent to such pressure application the release sheet is stripped from the magnetic layer. In this approach, the release sheet has a smooth surface which improves the smoothness of the stripped magnetic coating. Such an approach an improvement over the known calendaring and burnishing processes. Nonetheless problems arise if incomplete and partial transfer of the coating takes place. This tends to occur because the coating is still somewhat wet in nature. U.S. Pat. No. 4,312,898 discloses a process for making magnetic tapes in which a temporary release sheet is provided for transferring the magnetic layer to a permanent substrate and simultaneously provide a smooth surface for the magnetic coating.